Positive temperature coefficient (PTC) thermistor devices, comprising electrode/PTC element/electrode assemblies are well-known. The assemblies may be held together mechanically or by adhesion. The adhesion method has potentially the advantages of simplicity of design, ready transfer of heat at the contact areas, technological effectiveness, reliability, durability and economy. Examples of the relevant prior art are as follows:
U.S. Pat. Nos. 4,177,376, 4,330,703 and 4,543,474 describe a self-regulating heating article comprising a PTC layer and at least one layer of constant wattage output material at least partially contiguous therewith and preferably bound thereto by adhesive, which is activated and changes dimensions when the article is heated.
U.S. Pat. No. 4,419,564 describes use of an electrically and thermally conductive adhesive for bonding PTC elements to metallic elements in a self-regulating electric heater for use in an early fuel evaporation system for automotive engines.
U.S. Pat. No. 4,689,878 describes heat-generating resistance devices, for use in a crankcase heater, such devices including a PTC heater element adhesively bonded between a pair of metallic electrodes by means of an adhesive which is both thermally and electrically conductive.
In U.S. Pat. No. 4,899,032, there are described PTC ceramic resistors which are electrically coupled and mechanically fixed to metallic bodies (serving as current supply conduits) by an adhesive, and serve to heat a flowing medium in a heat exchanger.
U.S. Pat. No. 4,937,551 describes a PTC device wherein the PTC material is protected from degradation by mechanical (e.g. shear) forces by one or more adhesive masses in contact with both terminals of the device.
In U.S. Pat. No. 4,977,309, there is described an organic PTC device which includes an organic PTC thermistor sheet, on one main surface of which a pair of electrodes is formed and conductive adhesives are used to adhere a conductive sheet at each electrode, the assembly being covered by insulating film.
U.S. Pat. No. 5,239,163 describes an automobile air heater utilizing PTC tablets adhesively fixed to tubular heat sinks by means of a flexible electrically conductive adhesive.
U.S. Pat. No. 5,354,969 describes a PTC thermistor heater in which an electrode paste material containing conductive particles is printed and then baked on the surface of a PTC element to form a first electrode having a rough surface, which is brought into contact with a second electrode of heat radiating means by the conductive particles.
In U.S. Pat. No. 5,358,793, there is described a device having a PTC material bound on opposed sides thereof to conductive foils by a conductive adhesive having a thermal coefficient of expansion intermediate between that of the foil and that of the PTC material.
U.S. Pat. No. 5,499,087 describes a heat fixing device (for electrophotography) including a heating head formed by attaching a radiant plate to the radiant surface of a PTC element by use of a thermally conductive adhesive.
The entire contents of all of the above-stated U.S. Patents are deemed incorporated by reference herein.
In order to achieve the potential advantages mentioned above, however, it is desirable for the PTC thermistor devices to meet the following requirements:
1. Working temperature range is not less than about −55° C. to +300° C.
2. High thermal conductivity.
3. High thermal conductivity at the electrode/PTC element contact areas.
4. High dielectric strength and volume resistivity beyond the contact areas.
5. High resistance to repeated mechanical and thermal impacts in the range −55° C. to +300° C.
6. After hardening, adhesive resilience is maintained at the contact areas in order to compensate for mechanical tension at high temperatures.
7. Lack of corrosive attack on the cemented PTC thermistor devices.
8. The above-mentioned qualities 1-7 should be maintained for no less than 30,000 hours.
Known adhesives exhibit some but not all of these desired characteristics. For example, the high-temperature adhesive Ceramobond™ 5526, AREMCO PRODUCTS INC. Catalog M12 5/95, Bulletin No, M2, complies with the requirements as to working temperature range, dielectric properties and resistance to mechanical/thermal stresses (1, 4, 5), but does not comply with the other parameters (2, 3, 6, 8). Other adhesives, for example, AREMKO-BOND™ EPOXY 805. AREMCO PRODUCTS INC. Catalog V12 5/95, Bulletin No, M6, or AREMCO-SHlELD™ 615, AREMCO PRODUCTS INC. Catalog M12 5,95, Bulletin No. M17, comply with the above requirements as to working temperature range, heat conductivity and electrical conductivity/dielectric properties (1, 2, 4, 7), but not with the other parameters (3, 5, 6, 8). Some silicone adhesives, for example, DOW CORNING™ 3145 Silicone Adhesive-Gray, DOW CORNING CORPORATION, comply with the requirements as to working temperature range, dielectric properties, and resilience after hardening (1, 4, 5, 6, 7) but not with the other parameters (2, 3, 8). It is clear that none of the known adhesives combine all of the above-stated requirements, and in particular that none of them combine properties 3 and 8 with the other parameters, i.e. especially they do not provide high efficiency, reliability and durability in full measure.
A principal object of the present invention is to make possible the provision of an electrically and thermally conductive adhesive composition for cementing together PTC elements having metallized surfaces to metallic electrode elements, which combines all of the desired characteristics.
Another object of the invention is make possible the provision of such an adhesive which decreases resistance to thermal transfer and thereby increases the heat output of the resulting PTC thermistor devices.
Other objects of the invention will appear from the ensuing description.